Fig. 1 Diffraction of light for a slit with the length d. (code to produce this figure)

Have a look at Fig. 1 which tries to explain the diffraction phenomenon of a slit with the length d. At a distance a the diffraction pattern is drawn. The different distances, the distance between the first minima of the diffraction pattern, and the wave length are indicated by T-shaped arrows. This kind of arrays can be achieved in gnuplot with the following code.

Theads = 'heads size 0.5,90 front ls 201'
set arrow from -24,-2 to -24, 2       @Theads
set arrow from -22, 2 to -21.44,1.92  @Theads
set arrow from 1.5,-pi to 1.5,pi      @Theads
set arrow from -22,2.5*pi to 0,2.5*pi @Theads

Here, 90 is the relevant entry after size as it describes the opening angle of the arrow head.
In addition, an open circle is drawn to indicate the angle θ. This is achieved by specifying the opening angle for the circle object.

set object circle at -22,0 size 6 arc [-8:0]

set xlabel '{/Symbol P}'

But this produces a non italic Φ and it is not obvious how to get it italic. For normal letters the following will work

set xlabel '{/Helvetica-Italic P}'

and produces a P. But for the Symbol font Italic is not the right notation, instead you have to use Oblique:

set xlabel '{/Symbol-Oblique P}'

This will finally generate our desired italic Φ.

Update: in the meantime I have learned Helvetica-Italic is not part of the official postscript core fonts. Hence, use also Helvetica-Oblique.